1. Field of the Invention
The present invention relates to fastener devices and more particularly to a screw driver and screw head system.
2. Discussion of the Background of the Invention
Screws are one of the most widely used fastener devices. Generally they are made with a shank portion and a head portion. The shank portion is threaded, for example, with a metal or wood screw thread. In the common type of screw the head portion is dome-shaped and has a slot wih straight parallel walls. The screw driver used with such screw heads has a flat bottom edge and opposite slightly outwardly angled sides. To rotate the screw requires that the user exert pressure on the screw driver, both in a rotative direction and toward the screw head, to retain the blade in the slot. Frequently, when the screw is difficult to turn, the blade has a tendency to be forced out of the screw head slot by the rotative pressure on the screw driver.
The field of screw head designs and special forms of screw drivers is crowded with hundreds, if not thousands, of attempts to provide a screw head-screw driver system which is relatively low in cost and yet may provide a firm and secure turning force on the screw. For example, the "Phillips" (trademark) screw head and screw driver system utilizes a cruciform screw head impression to provide such an improved contact. A number of proposed screw head designs, some of which are discussed below, have suggested the use of a screw head slot having one or more undercut walls and a screw driver blade having an outwardly angled foot portion. That arrangement provides a cam action when rotative force is applied to the screw driver, which tends to draw (cam) the screw driver blade downwardly (inwardly) toward the screw head. Such cam action permits the user to apply only rotative force, and not pressure, along the screw axis to the screw driver and helps retain the screw driver and screw head in contact during the user's rotation of the screw driver.
The prior art includes the patents discussed below, as well as many others. However, there is still a need for a relatively low cost screw driver-screw head system which will permit the user to apply only rotative force and yet which will retain contact between the screw driver and screw head.
In U.S. Pat. No. 3,897,812 to Arnn for "Screw Driver" the blade has two opposed cut-away portions 24 formed by side ribs 26. The foot portion 28 has "wedge like bites 30" and "is generally isosceles trapezoidal in cross-section" and with "a flat base 32" (column 2, lines 38-42).
Another patent to Arnn, U.S. Pat. No. 3,923,088 for "Biting Screw Driver", has the cross-sectional view of the foot portion being curved (FIG. 3) or "generally isosceles trapezoidal" in shape with a flat base (FIGS. 6,10) and with a center blade bulbous portion (FIGS. 5-7) or cut-away portions, i.e., indentations (FIGS. 8-11). The screw driver is for use with standard screw heads whose slots have straight vertical walls, as shown in FIGS. 12,13 of the Arnn U.S. Pat. No. 3,923,088, and see its column 1, lines 23-26.
U.S. Pat. No. 3,695,321 to Garehime for "Cavity Head Screw and Driving Tool Therefor" shows a screw head having a complex geometry which may be difficult to produce at a low cost. It has a central cavity and two partial slots whose walls converge (as seen in top view) and are undercut (as seen in side view).
In U.S. Pat. No. 2,792,039 to Wing for "Slotted Screw Head and Driver Therefor Having Non-Burring Engagement", and in U.S. Pat. No. 2,677,985 to Vaughn for "Slotted Screw Head", a screw driver having an arcuate bottom edge fits in an arcuate slot having flat and parallel side walls which are then undercut to form four rounded undercut portions. The screw head may appear to be relatively complex and costly.
The U.S. Pat. Nos. 3,120,251 and 3,026,920, both to York for "Screwdriver", like the Wing patent, show a screw driver having an arcuate bottom edge which fits into a screw head slot having an arcuate bottom and which may have undercut side walls.
In U.S. Pat. No. 2,800,936 to West for "Cruciform Screw Driver" there is shown a type of cross recess having a center cavity and opposed undercut and overcut side walls.
In U.S. Pat. No. 2,684,094 to Lissy for "Nonslip Screw Driver and Screwhead", a screw driver has a protruding bottom "flared projections 17 and 18" which fit into undercut spaces 15,16 of the walls of a screw head slot. The projections act only when the screw driver is rotated in one direction (clockwise as in FIG. 2) and have no effect when the screw driver is reversed in direction.
"One-way" screws are employed when it is desired to screw in the screw using an ordinary screw driver, but removal using such a screw driver is not possible. For example, such screws may be used to fasten lock plates to doors. One type of "one-way" screw has a head without a slot, but divided into four sectors about a common center. Two opposed sectors are raised and have straight walls facing in the same rotative direction and about 360.degree. apart. The straight wall sectors are separated by sectors in the form of ramps. The screw may be inserted by the screw driver blade being turned clockwise and pushing on the two straight walls. When the screw driver is rotated counterclockwise it rides upward on the ramps and does not turn the screw.